Apical Oxygen Research Articles

Abnormal phase flip in the coherent phonon oscillations of Ca2RuO4

We employ an optical pump-probe technique to study coherent phonon oscillations in Ca$_2$RuO$_4$. We find that oscillation-amplitude of an $A_g$ symmetric phonon mode is strongly suppressed at 260 K, a putative transition point of orbital ordering. The oscillation also shows a gradual but huge change in its $phase$, such that the oscillation even flips over with a 180$^{\circ}$ change across the temperature. Density functional theory calculations indicate that the $A_g$ phonon has an eigenmode of octahedral distortion with conventional tilting along the $a$-axis and antipolar distortion of apical oxygen. Careful inspection of the lattice captures an unusually large antipolar distortion in low-temperature structures, which may play a crucial role for the phase transition at 260 K.

Apical Charge Flux-Modulated In-Plane Transport Properties of Cuprate Superconductors.

For copper-based superconductors, the maximum superconducting transition temperature T_{c,max} of different families measured from experiment can vary from 38K in La_{2}CuO_{4} to 135K in HgBa_{2}Ca_{2}Cu_{3}O_{8} at the optimal hole doping concentration. We demonstrate herein, using abinitio computations, a new trend suggesting that the cuprates with stronger out-of-CuO_{2}-plane chemical bonding between the apical anion (O, Cl) and apical cation (e.g., La, Hg, Bi, Tl) are generally correlated with higher T_{c,max} in experiments. We then show the underlying fundamental phenomena of coupled apical charge flux and lattice dynamics when the apical oxygen oscillates vertically. This triggers the charge flux among the apical cation, apical anion, and the in-plane CuO_{4} unit. The effect not only dynamically modulates the site energy of the hole at a given Cu site to control the in-plane charge transfer energy, but also can modulate the in-plane hole hopping integral simultaneously in a dynamic way by the cooperative apical charge fluxes.

Oxygen Reduction Effect on T’-Pr<sub>2-</sub><i><sub>x</sub></i>Ce<i><sub>x</sub></i>CuO<sub>4</sub> Nanopowders in the Underdoped Regime Studied by X-Ray Absorption near Edge Structure

This research is aimed to examine oxidation state of Copper (Cu) in both as-synthesized and reduced T’-Pr2-xCexCuO4 (T’-PCCO) with x = 0, 0.10, and 0.15 using Cu K-edge x-ray absorption near edge structure (XANES). The T‘-PCCO nanopowders were successfully synthesized by the chemically dissolved method with HNO3 as a dissolving agent continued by calcination at 1000°C for 15 h. The reduced T’-PCCO nanopowders were obtained by reduction annealing process at 700°C for 5 h under Ar gas atmosphere. The analyses of XANES spectra show that oxidation states of the Cu ions in all of the T'-PCCO nanopowders have values between +1 and +2. This indicates the existence of electron doping in the CuO2 planes, even in the undoped T’-structure. It is found that the oxidation states of the Cu ions change after reduction annealing depending on the existence of apical oxygen in the T'-structure. Based on the XANES analyses, it is revealed that the change of oxidation state is influenced by the presence of both electron and hole carriers in the two-carrier model of T’-structure.

Crystal structure and monoclinic distortion of glaserite-type Ba3MnSi2O8

Crystal structure and magnetic properties of glaserite-type Ba3MnSi2O8 were investigated using variable temperature neutron powder diffraction and magnetometry. At room temperature the composition is hexagonal and the crystal structure is best described by the P-3m1 space group (a~ 5.7 Å, c~ 7.3 Å) with the apical oxygen atom modelled on a split site. On cooling below ~ 250 K the structure undergoes a phase transition into a monoclinic C2/c form (√3ahex, ahex, 2chex, β~ 90°). Analysing diffraction data in terms of symmetry-adapted distortion modes suggests that the transition is primarily driven by increasing in-plane displacements of O1, which in turn results in the coupled tilting of [SiO4] and [MnO6] octahedra and in-plane displacements of Ba1 atoms. Magnetic susceptibility measurements and neutron powder diffraction data show no evidence for long-range magnetic ordering down to 1.6 K, although the development of magnetic diffuse scattering suggests that a magnetic transition may take place at lower temperature.

Cobalt spin states investigation of Ruddlesden-Popper La2−xSrxCoO4, using X-ray diffraction and infrared spectroscopy

Ruddlesden-Popper (RP) La2−xSrxCoO4 (x = 0.7, 0.9, 1.1, 1.3) were synthesized by a modified sol-gel route. The synthesized samples were characterized by XRD, FE-SEM technique, iodometric titration, FT-FIR spectroscopy and VSM analysis. The variation of Co-O bond length obtained from Rietveld refinements and the related peak shifts in the FT-FIR spectra showed that Co3+ has both LS and IS spin states. It is found that the population of Co3+ in the LS state decreases and IS state increases with increasing Sr concentration. Jahn-Teller effect was identified in the FT-FIR absorption bands, corresponding to the vibration modes of Co bonds with the apical and equatorial oxygen atoms. The ferromagnetic characteristic of La0.7Sr1.3CoO4, obtained by VSM, revealed the increase of Co3+ IS population by Sr doping, in agreement with XRD and FT-FIR results. The shift and split of the IR absorption bands with the change in the structural parameters are proposed to be an applicable tool to probe the spin states and Jahn-Teller effect in La2−xSrxCoO4 compounds.

Magnon Splitting Induced by Charge Transfer in the Three-Orbital Hubbard Model.

Understanding spin excitations and their connection to unconventional superconductivity have remained central issues since the discovery of cuprates. Direct measurement of the dynamical spin structure factor in the parent compounds can provide key information on important interactions relevant in the doped regime, and variations in the magnon dispersion have been linked closely to differences in crystal structure between families of cuprate compounds. Here, we elucidate the relationship between spin excitations and various controlling factors thought to be significant in high-T_{c} materials by systematically evaluating the dynamical spin structure factor for the three-orbital Hubbard model, revealing differences in the spin dispersion along the Brillouin zone axis and the diagonal. Generally, we find that the absolute energy scale and momentum dependence of the excitations primarily are sensitive to the effective charge-transfer energy, while changes in the on-site Coulomb interactions have little effect on the details of the dispersion. In particular, our result highlights the splitting between spin excitations along the axial and diagonal directions in the Brillouin zone. This splitting decreases with increasing charge-transfer energy and correlates with changes in the apical oxygen position, and general structural variations, for different cuprate families.

Enhanced superconducting properties of Ti doped (Cu0.5Tl0.5)Ba2(Ca2−xTix)Cu3O10−δ samples

Ti-doped (Cu0.5Tl0.5)Ba2(Ca2−xTix)Cu3O10−δ (x = 0, 0.25, 0.50, 0.75, 1.0) superconductors have been synthesized by solid state reaction method. The prepared samples are characterized by XRD, electrical resistivity, AC susceptibility and FTIR techniques. XRD analysis showed that the lattice parameters are marginally altered but there is no substantial change with the doping of Ti. All the samples are found with orthorhombic crystal structure following PMMM space group. Tc(R = 0) is enhanced with the doping of Ti except for x = 1.0 sample. The magnitude of diamagnetism in the AC-susceptibility measurements is enhanced up to x = 0.5 and decreased beyond. The FTIR measurements show that the apical oxygen phonon modes are softened. The planar oxygen phonon modes are softened but their intensity is raised with the enhanced Ti contents. This softening of the planar oxygen phonon modes may be arising due to the heavier Ti ions (47.9 amu) at the lighter Ca (40.07 amu) sites. Variation in the lattice parameters in the XRD data and shifting of various oxygen phonon modes in the FTIR data show that Ti is incorporated at Ca sites in the unit cell. Increase in the superconducting properties up to certain doping level (x = 0.75) may be arising due to the improved interplane coupling caused by the smaller sized Ti at Ca sites. While the decrease in Tc and magnitude of diamagnetism beyond x = 0.75 and x = 0.5 respectively, is attributed to the suppressed density of a particular type of phonons required for optimum superconductivity. This suppression in the density of the desired phonons is brought about by the substitution of heavier Ti ions at the lighter Ca sites. This study signifies the role of electron–phonon interaction in mechanism of high Tc superconductivity.

Metal-Insulator Transition in Copper Oxides Induced by Apex Displacements

High temperature superconductivity has been found in many kinds of compounds built from planes of Cu and O, separated by spacer layers. Understanding why critical temperatures are so high has been the subject of numerous investigations and extensive controversy. To realize high temperature superconductivity, parent compounds are either hole-doped, such as {La$_{2}$CuO$_4$} (LCO) with Sr (LSCO), or electron doped, such as {Nd$_{2}$CuO$_4$} (NCO) with Ce (NCCO). In the electron doped cuprates, the antiferromagnetic phase is much more robust than the superconducting phase. However, it was recently found that the reduction of residual out-of-plane apical oxygens dramatically affects the phase diagram, driving those compounds to a superconducting phase. Here we use a recently developed first principles method to explore how displacement of the apical oxygen (A-O) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small displacements of the apical oxygens drive the compound to a metallic state through a localization/delocalization transition, with a concomitant maximum $d$-wave order parameter at the transition. We address the question whether NCO can be seen as the limit of LCO with large apical displacements, and elucidate the deep physical reasons why the behaviour of NCO is so different than the hole doped materials. We shed new light on the recent correlation observed between T$_c$ and the charge transfer gap, while also providing a guide towards the design of optimized high-Tc superconductors. Further our results suggest that strong correlation, enough to induce Mott gap, may not be a prerequisite for high-Tc superconductivity.

Low frequency dielectric response of Cu0.5Tl0.5Ba2Ca2Cu3-xMxO10-δ (M = Cd, Zn, Ni; x=0, 1.5) superconductors

Cu0.5Tl0.5Ba2Ca2Cu3-xMxO10-δ (M = Cd, Zn, Ni; x = 0, 1.5) samples are synthesized by the two-step solid-state reaction method and extensively characterized by low frequency dielectric measurements. These samples have shown orthorhombic crystal structure in which the length of a-axis increases, but b & c-axes lengths suppress with the incorporation of Cd, Zn, Ni in the final compound. The Tc(onset) and Tc(R = 0) suppress with the doping of Cd whereas its values increase with the incorporation of Zn and Ni in the final compound. In ac-susceptibility measurements a suppression in the onset temperature of superconductivity observed in all doped samples, however, the maximum decrease is observed in Cd-doped samples. The vibrations of apical oxygen phonon modes of type Tl-OA-Cu(2) are hardened but the peak positions of other modes stay unchanged with the doping of Cd, Zn, Ni in the final compound. In the frequency dependent dielectric measurements the real (ε/) and imaginary (ε//) parts of the dielectric constants are suppressed with the increase of applied measurement frequency, however, their absolute values increase with the decrease of measurement temperature. In comparison with un-doped samples the absolute values of real ε/ and imaginary parts ε// of dielectric constants suppress with the doping of Cd, Zn, Ni in the final compound; a maximum suppression is observed in Cd-doped samples. We have proposed from these results that doped Cd atoms in CuO2/CdO2 planes produce an-harmonic oscillations, that in turn promotes the decrease in density of Cooper-pairs; it is reflected in the decrease of the dielectric constants of the final compound.Novel shift in dielectric loss (tanδ) from the negative to the positive value observed in the frequency region between 800 Hz and 1000 Hz in all Cu0.5Tl0.5Ba2Ca2Cu3-xMxO10-δ (M = Cd, Zn, Ni; 1.5) samples is attributed to be arising from the activation of various interfacial polarizations at different frequencies. An enhanced ac-conductivity jump is observed in pristine, Zn and Ni-doped samples which is altogether absent in Cd-doped samples providing unequivocal evidence of generation of an-harmonic oscillations and their pair breaking effects.

Synthesis, Crystal Structure, and Optical Properties of Layered Perovskite Scandium Oxychlorides: Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2.

We report the successful synthesis of three new Ruddlesden-Popper-type scandium oxychloride perovskites, Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2, by conventional solid-state reaction. Small single crystals of Sr2ScO3Cl were obtained by a self-flux method, and the crystal structure was determined to belong to the tetragonal P4/ nmm space group ( a = 4.08066(14) Å, c = 14.1115(8) Å) by X-ray diffraction analysis. The scandium center forms a ScO5Cl octahedron with ordered apical oxygen and chlorine anions. The scandium cation, however, is shifted from the position of the octahedral center toward the apical oxygen anion, such that the coordination geometry of the Sc cation can be effectively viewed as an ScO5 pyramid. These structural features in the oxychloride are different from those of octahedral ScO5F coordinated with a partial O/F anion order at the apical sites in the oxyfluoride Sr2ScO3F. Rietveld refinements of the neutron powder diffraction data of Sr3Sc2O5Cl2 ( I4/ mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and Ba3Sc2O5Cl2 ( I4/ mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO5 pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in Sr2ScO3Cl with the exception that the ScO5 pyramids form double layers by sharing the apical oxygen. Density functional calculations on Sr2ScO3Cl indicate the strong covalency of the Sc-O bonds but almost nonbonding interaction between Sc and Cl ions.

TCNQ molecular semiconductor of the Cu(II)TAAB macrocycle: Optical and electrical properties

The present study reports the doping of a semiconducting molecular material through the formation of hydrogen bonds between the macrocycle Cu(II)(TAAB) and the electronic acceptor TCNQ. According to density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) analysis, the doped compound has the shape of a distorted square pyramid, with four nitrogen atoms in the equatorial position and the apical oxygen atom from the water ligands. These water molecules can generate strong hydrogen bonds with TCNQ and the TAAB metallic complex. Thin films of copper molecular material were obtained through high vacuum evaporation and were structurally characterized by IR spectroscopy, EPR and scanning electron microscopy (SEM). Additionally, the absorption coefficient (α) and photon energy (hν) were calculated from UV–vis spectroscopy and used to determine the optical activation energy in each film, from which its semiconducting behavior was established. An important aspect to consider is that the presence of hydrogen bonds is essential to establish the semiconducting nature of these species; this chemical behavior, as well as the resulting electronic mobility, have been studied by DFT theoretical calculations, which reinforce the experimental conclusion of a relationship between Cu(II)TAAB and TCNQ moieties generated by a weak bond. Finally, I–V characteristics have been obtained from a glass/ITO/doped molecular semiconductor/Ag device using Ag and ITO electrodes. Results for the copper-based device show that, at low voltages, the conduction process is of an ohmic nature while, at higher voltages, space-charge-limited current (SCLC) is found. It is highly probable that the doping effect in TCNQ favors electronic transport due to the formation of conduction channels caused by dopant-favored anisotropy.

(Nd/Pr)2NiO4+δ: Reaction Intermediates and Redox Behavior Explored by in Situ Neutron Powder Diffraction during Electrochemical Oxygen Intercalation.

Oxygen intercalation/deintercalation in Pr2NiO4+δ and Nd2NiO4+δ was followed by in situ neutron powder diffraction during electrochemical oxidation/reduction, in a dedicated reaction cell at room temperature. For both systems three phases, all showing the same line width, were identified. The starting phases Pr2NiO4.23 and Nd2NiO4.24, considered with an average orthorhombic Fmmm symmetry, although both show a slight monoclinic distortion, get reduced in a two-phase reaction step to tetragonal intermediate phases with 0.07 ≤ δ ≤ 0.10 and P42/ ncm space group, which on further reduction transform, again in a two-phase reaction step, toward the respective stoichiometric (Pr/Nd)2NiO4.0 phases, with Bmab space group. Electrochemical oxidation does, however, not proceed fully reversibly for both cases: while the reoxidation of Nd2NiO4+δ is limited to the tetragonal intermediate phase with δ = 0.10, the homologous Pr2NiO4+δ can be reoxidized up to δ = 0.17, showing orthorhombic symmetry. For the intermediate tetragonal phase, we were able to establish for Pr2NiO4.09 a complex anharmonic displacement behavior of the apical oxygen atoms, as analyzed by single-crystal neutron diffraction and maximum entropy analysis, in agreement with a low- T diffusion pathway for oxygen ions, activated by lattice dynamics.

X-ray absorption spectroscopy study of annealing process on Sr1–xLaxCuO2 electron-doped cuprate thin films

The superconducting properties of Sr1–xLaxCuO2 thin films are strongly affected by sample preparation procedures, including the annealing step, which are not always well controlled. We have studied the evolution of Cu L2,3 and O K edge x-ray absorption spectra (XAS) of Sr1–xLaxCuO2 thin films as a function of reducing annealing, both qualitatively and quantitatively. By using linearly polarized radiation, we are able to identify the signatures of the presence of apical oxygen in the as-grown sample and its gradual removal as a function of duration of 350 °C Ar annealing performed on the same sample. Even though the as-grown sample appears to be hole doped, we cannot identify the signature of the Zhang-Rice singlet in the O K XAS, and it is extremely unlikely that the interstitial excess oxygen can give rise to a superconducting or even a metallic ground state. XAS and x-ray linear dichroism analyses are, therefore, shown to be valuable tools to improving the control over the annealing process of electron doped superconductors.

Oxygen vacancy induced structural evolution of SrFeO3−x epitaxial thin film from brownmillerite to perovskite

We investigated SrFeO$_{3-x}$ thin films on a SrTiO$_3$ (001) substrate prepared via pulsed laser epitaxy using an optical spectroscopy technique. The oxygen vacancy level ($x$) was controlled by post-annealing processes at different oxygen partial pressures. We achieved a brownmillerite(BM) structure at $x =$ 0.5 and observed the evolution of the crystal structure from BM into perovskite(PV) as the oxygen concentration increased. We observed the evolution of infrared-active phonons with respect to the oxygen concentration, which was closely related to the structural evolution observed via X-ray diffraction. We identified the phonons using the shell-model calculation. Furthermore, we studied temperature-dependent behaviors of the phonon modes of three representative samples: PV, and two BMs (BM$_{\mathrm{oop}}$ and BM$_{\mathrm{ip}}$) with different orientations of the oxygen vacancy channel. In the BM$_{\mathrm{oop}}$ sample, we observed a phonon mode, which exhibited an unusual red-shift with decreasing temperature; this behavior may have been due to the apical oxygen instability in the FeO$_6$ octahedron. Our results provide important information regarding the ionic conduction mechanism in SrFeO$_{3-x}$ material systems.

NaCl-CsCl structural transition in Sr2PdO3 and Sr2TiO4

First principles investigations are conducted to explore the NaCl to CsCl phase transition of Sr2PdO3 and to predict the similar transition of the Ruddlesden-Popper type layered perovskite Sr2TiO4. Our results established that a first-order phase transition from Immm phase to Ammm phase of Sr2PdO3 occurs at around 30.6 GPa, being in good accordant with the experimental data (29 GPa). More interesting, the analogous topotactic structural phase transition from I4/mmm phase to orthorhombic Cmcm one is predicted to take place at about 66.6 GPa in Sr2TiO4, and associated with the increase of oxygen anion coordination number around strontium cation from nine to ten. Compared to the case of Sr2PdO3 with MO4 square planar motif, the influence of the occupancy of the vacant apical oxygen on the critical transition pressure has been deliberated. As far as we know, the I4/mmm→Cmcm transition in Sr2TiO4 has not been subject to detailed experimental high pressure studies yet. Furthermore, Sr2TiO4 is a prototypical Ruddlesden-Popper type perovskite, thus, this typical phase transition is probably ubiquitous in many compounds. We hope our results will motivate further studies, theoretical and/or experimental, on the high pressure behavior of the Ruddlesden-Popper series of layered perovskites.

New superconducting materials discovered via high pressures technology

In this study, we primarily select the author′s research to briefly introduce the various superconducting materials that are synthesized using high pressures. Superconducting systems included are ranging from cuprates, iron-based materials, to topological compounds. They are fabricated using: ″self-high-pressure oxidization″, which is produced using the chemical composition of the synthesized material itself rather than using traditional alien oxidizers; ″inner pressure effects″, which are generated using the atomic size changes; and a ″pure doping mechanism″ that is induced using a pressure-induced charge transfer, respectively. We focus on three categories of superconductors in our investigation. (1) Apical oxygen ordered cuprate superconductors that reach the highest T c for the monolayered cuprate superconductors, Cu12( n −1) n homologues series cuprate superconductors having a T c of up to 123 K, and a Cl2( n −1) n homologues series cuprate superconductor that is the first halogen-containing superconducting system having a T c higher than liquid nitrogen: These cuprate superconductors comprise only copper and alkaline earth metal oxides, which are the simplest reagents that are necessary to form high T c cuprates. However, those high pressure synthesized materials exhibit very high T c that usually can only be realized in cuprates superconductors with complex compositions if prepared at ambient condition such as YBCO123, Bi22( n −1) n , or Hg12( n −1) n . The new superconductors exhibit excellent critical electrical current density as a function of temperature, especially at high temperatures. They show superior properties than the Bi/Hg cuprate homologues superconductors and are comparable to the classical Y123. (2) The ″111″ iron-based superconducting system: This system features neutral-charged cleaved fresh surfaces that are perfect for experimental studies based on angle resolved photoemission electron spectroscopy (ARPES) or scanning transmission microscopy (STM). Compared with other important iron-based superconductors, this unique feature facilitates the unambiguous detection of intrinsic properties without surface reconstruction. Studies on LiFeAs, which are representative members of the ″111″ system, reveal the non-Fermi surface nesting characteristics that are in sharp contrast to the observations in previous iron-based superconducting systems. This study indicates that Fermi surface nesting is not necessary for the generation of superconductivity in iron-based materials. This is a critical step to understand the mechanisms of iron-based superconductors. (3) Pressure-induced superconductivity in the topological compound Bi2Te3: this is the first superconductor in a topological compound generated by applying pressure and without using chemical doping.

Crystal and Electronic Structure of High-Temperature Superconductive Layered Cuprates in Temperature Interval 100–300 K

For two HTSC materials, Y1−x Ca x Ba2Cu3 O 6.7 and Tl0.8Hg0.2Ba2Ca2Cu3 O 8.15, their crystal structure has been studied earlier in the temperature interval 100–300 K. It is shown that for both compounds, the height of CuO5 pyramids has a minimum in temperature interval from ∼ 150 to ∼ 250 K. The electronic structure is calculated using first principles based on the obtained data of crystal structure. It is revealed that a decrease in the CuO5 pyramid height leads to the appearance of the electronic state density peak with an energy of 0.3–0.4 eV under Fermi level. Localized states of barium and apical oxygen make the main contribution to this peak. By reduction the temperature below ∼ 150 K, these localized electronic states disappear, and under continuous cooling, the superconductivity appears.

Crystal structure, electrical and electrochemical properties of Cu co-doped Pr1.3Sr0.7NiO4+δ mixed ionic-electronic conductors (MIECs)

Room temperature crystal structure, electrical properties and electrochemical properties in the temperature range 25–700 °C of Cu co-doped Pr1.3Sr0.7NiO4+δ prepared by acetate combustion is investigated from intermediate temperature solid oxide fuel cell cathode viewpoint. The Pr1.3Sr0.7Ni1−xCuxO4+δ (PSNCO) solid solutions have a tetragonal I4/mmm K2NiF4-type structure which consists of a (Pr1.3Sr0.7) (Ni1.xCux)O3 perovskite unit and a (Pr1.3Sr0.7)O rock salt unit in the whole compositional range 0 ≤ x ≤ 0.4. A reduction in bond-length of Ni/Cu-O resulting from PSNCO lattice contraction eases hop of small polaron from Ni3+ to Ni2+/Cu2+ in (Ni1−xCux)-O layer with low activation energy, which increases electron conductivity. The maximum electronic conductivity (σ = Ω cm−1) with minimum activation energy (Ea = eV) is observed at x = 0.3. Lattice expansion along c-direction owing to Cu2+ doping facilitates hop of O2− from its occupied interstitial site (O3) to nearby equivalent site assisted by anisotropic thermal motion of apical oxygen O2 resulting in increase in ionic conductivity. The minimum polarization resistance value (Rp = 0.13 (2) Ω cm2) and activation energy (Ea = 1.321 (5) eV) at x = 0.3 is attributed to high electronic and ionic conductivities compared to other compositions. Complex impedance spectroscopy studies suggest that the ORR is co-limited by O2− diffusion and O2 surface exchange.

Synchrotron X-ray Diffraction and High-Pressure Electrical Resistivity Studies for High-Tc Candidate Nd3.5Sm0.5Ni3O8

Ln4Ni3O8 (Ln = La, Nd, Sm) has attracted much attention as a candidate for high-Tc superconductor due to its close structural and electrical similarities with high-Tc cuprates. However, Ln4Ni3O8 is not a superconductor and shows semiconducting behavior. Our recent work has revealed that Nd3.5Sm0.5Ni3O8 displays metallic behavior down to 20–40 K upon intercalation and subsequent deintercalation treatments with sulfur, followed by a weak semiconducting tendency at lower temperatures. A synchrotron X-ray diffraction experiment suggests that the structural change induced by sulfur treatment can be explained electrostatically by the removal of additional apical oxygen. High-pressure electrical resistivity measurements up to 8 GPa on a metallic sample show the enhancement of the semiconducting tendency at low temperatures, suggesting that the removal of additional apical oxygen is not totally completed under the present conditions of sulfur treatment.

Crystal Structure of (Lu1−x Ca x )Ba2Cu3O7−z (x = 0, 0.2) Superconducting Compounds Studied by Neutron Powder Diffraction

The crystalline structures of two polycrystalline samples of (Lu1−x Ca x )Ba2Cu3O7−z with x = 0 (pure sample) and x = 0.2 (Ca-doped sample) have been analyzed by using neutron powder diffraction techniques to address the structural mechanisms associated with changes in superconducting behavior induced by Ca doping. The pure and Ca-doped samples exhibit superconductivity around 87 and 32 K, respectively. The room-temperature thermoelectric power measurements indicate that both the x = 0 and 0.2 samples are in overdoped state. The Rietveld refinements of the neutron diffraction data collected at room temperature for the pure and Ca-doped samples show that the higher level of oxygen vacancies is located on both the CuO2 planes and the apical oxygen sites for the pure sample compared to those of the Ca-doped sample. The present results suggest that the decrease in T c observed in the Ca-doped sample is mainly caused by the hole overdoping effect rather than by oxygen vacancies in the CuO2 planes.